Recently, the development of the backbone network communication line as mainly focusing on the optical fiber communication technology is proceeding steadily, in such a situation, the electronic wirings in the information terminal are becoming a bottleneck. Against such background, instead of the conventional electric circuit substrate in which all signal transmissions are made by using the electric signal, the optoelectronic composite substrate (optical waveguide device) of the type that transmits high-speed parts by the light has been proposed, in order to compensate the limit of transmission speed of the electric signal.
In the case that the light emitting element and the light receiving element of the surface mounted type are mounted to the optical waveguide, the light path conversion inclined surface that is inclined to intersect with the light propagation direction at an angle of 45° is formed at both ends of the optical waveguide, and then the light path conversion mirror is constructed to contact with the light path conversion inclined surface, so that the light path conversion is made.
In Patent Literature 1 (Japanese Laid-Open Patent Publication No. 2008-281816), it is set forth that the optical waveguide main body has the first area that a light signal is transmitted and the second area that is provided on both sides of the first area and does not contribute to a transmission of the light signal, and the penetration vias are provided in the second areas in the optical waveguide main body and also the first area of the optical waveguide main body is formed to protrude upward from both second areas of the optical waveguide main body.
Also, in Patent Literature 2 (Japanese Laid-Open Patent Publication No. 2010-39082), it is set forth that the optical circuit substrate is constructed by forming the printed wiring substrate, the optical waveguide, the light path converting portion, and the second cladding layer covering this light path converting portion on the supporting substrate, and then by peeling the supporting substrate.
As explained in the column of the related art described later, in forming the optical waveguide on the wiring substrate, the core layer is obtained by patterning the photosensitive resin layer based on the contact exposure system.
However, because of the influence of the connection pads of the wiring substrate, a level difference is caused in the photosensitive resin layer that is used to obtain the core layer. Therefore, it becomes difficult to contact the whole of the photomask to the photosensitive resin layer.
Accordingly, primary performance of the contact exposure cannot be fully drawn out. As a result, a pattern width of the core layer is varied beyond the design specification, and furthermore in some cases the resolution failure may occur, so that such a problem arises that the optical waveguide cannot be formed with good yield.